Ultraviolet Spectrophotometry in Residue Analysis; SpectraâStructure

7 Ultraviolet Spectrophotometry in Residue Analysis; Spectra-Structure Correlations O S M A N M. ALY and S. D .

FAUST

I.

Downloaded by FUDAN UNIV on January 16, 2017 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0104.ch007

Department of Environmental S c i ences, Rutgers University, N e w Brunswick, N. J. 08903

Ultraviolet

dues. A correlation ture of several correlation chromophore analytical

between provide

present procedures.

conjunction

bands

the UV spectrum

is discussed. clues

and may a limitation

in this region.

the

such

type to

of

when

taken

by IR, NMR, A discussion

for the analysis

level is also

in the in and

may lead to structural analyst.

of

design

of many groups

on interpretations However,

that have been utilized

cides on the submicrogram

analyst

struc-

of

general

the

obtained

UV spectra

posals of value to the pesticide methods

help

tool resi-

and the

Knowledge

about

with the information

mass spectroscopy,

a valuable of pesticide

The transparency

the near UV imposes absorption

is considered

the identification

pesticides

may

SUFFET

Drexel Institute of Technology, P h i l a delphia, P a .

spectrophotometry

as an aid for confirming

H.

proof

of

the

pesti-

presented.

l t r a v i o l e t s p e c t r o p h o t o m e t r y has b e e n , for some t i m e , one of the most ^

v a l u a b l e tools of p e s t i c i d e r e s i d u e analysis. A n y c o m p o u n d w i t h a

sufficiently w e l l - d e f i n e d a n d intense a b s o r p t i o n spectra is p o t e n t i a l l y d e tectable b y this s p e c t r o p h o t o m e t r i c means. I n some cases, s u i t a b l e c h r o mophores Gunther

m a y b e p r o d u c e d b y c h e m i c a l transformations. (1)

Blinn

h a v e r e v i e w e d the different p r o c e d u r e s t h a t h a v e

u t i l i z e d for r e s i d u e analysis at the m i c r o g r a m l e v e l .

UV

and been

spectropho-

t o m e t r y is c o n s i d e r e d one of t h e v a l u a b l e aids for i d e n t i f i c a t i o n of o r g a n i c pesticides. A l t h o u g h the u l t r a v i o l e t d a t a m a y not p r o v i d e the m u l t i t u d e of i n f o r m a t i o n that c a n b e g a i n e d f r o m other s p e c t r o p h o t o m e t r i c m e t h o d s s u c h as I R , N M R , or mass s p e c t r o m e t r y , it is often possible to r e v e a l subtleties of structure w h i c h these t e c h n i q u e s cannot r e v e a l

(2,3,4).

95

Biros; Pesticides Identification Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

96

PESTICIDES I D E N T I F I C A T I O N

D u r i n g t h e p a s t d e c a d e , t r e m e n d o u s advances h a v e b e e n a c h i e v e d i n the d e v e l o p m e n t of e m p i r i c a l correlations b e t w e e n the s t r u c t u r e of o r g a n i c m o l e c u l e s a n d t h e i r u l t r a v i o l e t s p e c t r a (4, 5 ) .

This paper dis-

cusses the c o r r e l a t i o n b e t w e e n the s p e c t r a of some pesticides i n the n e a r u l t r a v i o l e t r e g i o n (190 to 400 fi) a n d t h e i r s t r u c t u r e . S u c h i n f o r m a t i o n w o u l d b e of v a l u e to the r e s i d u e c h e m i s t f o r q u a l i t a t i v e a n d q u a n t i t a t i v e purposes.

A l s o , the a p p l i c a t i o n of U V s p e c t r o p h o t o m e t r y

for

residue

analysis o n the s u b m i c r o g r a m l e v e l is p r e s e n t e d .


Electronic Transitions A b s o r p t i o n of q u a n t a of r a d i a t i o n i n t h e u l t r a v i o l e t r e g i o n results i n a s p e c t r a l t r a n s i t i o n i n w h i c h t h e electrons of m o l e c u l e s are e x c i t e d f r o m t h e i r g r o u n d state to h i g h e r e n e r g y levels. T h e e n e r g y of a p a r t i c l e w h i c h is c o n f i n e d to a v e r y s m a l l r e g i o n is r e s t r i c t e d to c e r t a i n values.

These

energies are s u b j e c t e d to q u a n t u m restrictions w h i c h a l l o w the m o l e c u l e ( o r e l e c t r o n s ) to h a v e o n l y c e r t a i n energies. G e n e r a l l y , these restrictions are i n c r e a s i n g l y i m p o r t a n t as the r e g i o n i n w h i c h the p a r t i c l e is free to m o v e b e c o m e s s m a l l e r . T h a t is, of the w h o l e r a n g e of energies,

fewer

a n d m o r e w i d e l y s p a c e d ones are a l l o w e d as the p a r t i c l e m o t i o n is m o r e r e s t r i c t e d . T h e r e f o r e , the electrons w h i c h are confined to the v o l u m e , or p e r h a p s , to a p a r t of the v o l u m e of the m o l e c u l e are subject to significant q u a n t u m restrictions or w i l l b e q u a n t i z e d . T h e s e restrictions a l l o w the p r o p e r t i e s of m o l e c u l e s to b e s t u d i e d b y the m e t h o d s of (6).

spectroscopy

T h e e n e r g y of a q u a n t u m of r a d i a t i o n is c a l c u l a t e d f r o m P l a n k s

equation AE

=

Kv

(1)

w h e r e A E is the energy of a q u a n t u m ( e r g s ) , K is P l a n k s constant (6.62 10"

27

X

e r g - s e c ) , v is the f r e q u e n c y of w a v e m o t i o n i n c y c l e s / s e c o n d , a n d

(2) w h e r e c is the v e l o c i t y of l i g h t ( 3 X

10

10

c m / s e c ) a n d A is the w a v e l e n g t h

in cm. T h e e n e r g y of a q u a n t u m of r a d i a t i o n h a v i n g w a v e l e n g t h s of 700, 400, 200, a n d 150 is, therefore, 2.85 X 13.24 X

10"

12

1 0 , 4.97 X 1 2

10 , 9.9 X 12

10" , a n d 12

ergs, r e s p e c t i v e l y .

W h e n a m o l e c u l e is i r r a d i a t e d b y h e t e r o c h r o m a t i c l i g h t , it absorbs o n l y the protons w h o s e energies are e q u a l to that r e q u i r e d for p e r m i s s i b l e e n e r g y transitions w i t h i n the m o l e c u l e .

F o r most l a r g e m o l e c u l e s , the

e l e c t r o n i c a b s o r p t i o n b a n d is v e r y c o m p l i c a t e d , a n d a r a t h e r b r o a d a b s o r p -


7.

A L Y , FAUST, SUFFET

Spectra-Structure

97

Correlations

t i o n b a n d w i t h l i t t l e o r no d e t a i l is o b t a i n e d .

T h i s is p a r t i c u l a r l y t r u e

w h e n the m a t e r i a l is s t u d i e d i n s o l u t i o n . A l t h o u g h a b s o r p t i o n of u l t r a v i o l e t l i g h t results i n the e x c i t a t i o n of electrons f r o m t h e i r g r o u n d state, the n u c l e i w h i c h the electrons h o l d together d e t e r m i n e the s t r e n g t h of the b i n d i n g . T h u s , the c h a r a c t e r i s t i c e n e r g y of transition, a n d h e n c e the w a v e l e n g t h of a b s o r p t i o n , is r e l a t e d to the e l e c t r o n i c s t r u c t u r e of a g r o u p of atoms w i t h i n the m o l e c u l e . g r o u p p r o d u c i n g a b s o r p t i o n is c a l l e d a c h r o m o p h o r e .

The

S t r u c t u r a l changes

affecting a c h r o m o p h o r e c a n b e e x p e c t e d to m o d i f y its a b s o r p t i o n .


T h e t h r e e m a j o r c o n t r i b u t o r s to e l e c t r o n i c s p e c t r a are the s i n g l e b o n d ( a - e l e c t r o n s ) , the m u l t i p l e b o n d ( ^ - e l e c t r o n s ) , a n d the u n s h a r e d e l e c t r o n pair

(n-electrons). C o m p o u n d s h a v i n g o n l y a - v a l e n c y electrons are s a t u r a t e d a n d

do

not a b s o r b i n the n e a r u l t r a v i o l e t r e g i o n . A l l the electrons of the m o l e c u l e are i n v o l v e d i n s i n g l e b o n d s , a n d t h e y cannot b e r e a r r a n g e d to a n e x c i t e d state w i t h o u t d i s r u p t i o n of the m o l e c u l a r b o n d i n g .

E x c i t a t i o n of

these

electrons r e q u i r e s h i g h - e n e r g y photons i n the f a r u l t r a v i o l e t r e g i o n (A 150 fi).

/

t r a n s i t i o n o f a n n - e l e c t r o n to t h e r i n g o r b i t a l ( 4 ) .

Acidification of a solution of compound containing the pyridine nucleus r e m o v e s this l o n g - w a v e l e n g t h b a n d f r o m t h e s p e c t r u m .

Substitution of

the p y r i d i n e r i n g i n t h e a o r /3 positions results i n a shift o f t h e 257-it


b a n d to longer wavelengths.

N i c o t i n e ( F i g u r e 7 ) a n d anabasine are

t y p i c a l examples o f t h e ^ - s u b s t i t u t e d p y r i d y l c h r o m o p h o r e w h i c h absorbs at 262 ii. P a r a q u a t ( a q u e o u s ) absorbs at 256 /A. I n this m o l e c u l e are n o n o n b o n d e d

there

electrons b u t t h e y a r e i n v o l v e d i n t h e c a t i o n f o r m a -

t i o n , a n d t h e s p e c t r u m is v e r y s i m i l a r t o t h a t o f a n a c i d i c s o l u t i o n o f

200

240 WAVELENGTH,

280

320

mjj

Figure 7. Ultraviolet absorption spectra of pyridine, (in ethanol), paraquat, ana diquat (in water)

nicotine



110

PESTICIDES

IDENTIFICATION

WAVELENGTH,

Figure

8. Ultraviolet absorption spectra of simazine and atrazine (in water)

p y r i d i n e . T h e s p e c t r u m o f d i q u a t , o n the other h a n d , shows a d r a m a t i c b a t h o c h r o m e shift t o 308 tt s i m i l a r t o t h a t o b s e r v e d w i t h the a - t e r - a m i n o p y r i d y l derivatives

{24).

St/m-TRiAZiNE CHROMOPHORE.

T h e s p e c t r u m of s i m a z i n e a n d a t r a z i n e

i n w a t e r ( F i g u r e 8 ) shows a w e a k b a n d at 263 tt w h i c h is t o b e c o m p a r e d w i t h the IT — » TT* (255 tt) b a n d o f b e n z e n e . A n o t h e r intense b a n d at 220 tt is o b s e r v e d w h i c h is c h a r a c t e r i s t i c o f a l l t h e s u b s t i t u t e d st/ra-triazines (Table I X ) .

Ultraviolet Micro Spectrophotometry as an Aid to the Identification of Pesticide Residues R e c e n t l y Suffet ( 2 5 ) a n d F a u s t a n d Suffet ( 2 6 ) r e p o r t e d a n i n t e n sive s t u d y o n t h e s e p a r a t i o n a n d i d e n t i f i c a t i o n o f t h e p h o s p h a t e

ester

pesticides p a r a t h i o n , d i a z i n o n , a n d f e n t h i o n a n d t h e i r d e g r a d a t i o n p r o d ucts. A s u m m a r y o f a m i c r o s p e c t r o p h o t o m e t r i c u l t r a v i o l e t t e c h n i q u e that w a s u t i l i z e d as a n a i d i n i d e n t i f y i n g these c o m p o u n d s i s p r e s e n t e d here.


7.

A L Y , FAUST, SUFFET

Spectra-Structure

A B e c k m a n D K - 2 A spectrophotometer ard benzene vapor spectrum.

111

Correlations

was calibrated with a stand-

T h e spectrophotometer

w a s adjusted for

the h i g h e s t r e s o l u t i o n w i t h a t o l e r a b l e signal-to-noise r a t i o . S i n c e h i g h r e s o l u t i o n d e m a n d s a n a r r o w slit w i d t h , i n s t r u m e n t s e n s i t i v i t y w a s set as h i g h as possible. A s a m a x i m u m s i g n a l response w a s n e e d e d , a l o w t i m e constant w a s set, a n d a r e l a t i v e l y s l o w s c a n n i n g s p e e d w a s u s e d . A 1-cm B e c k m a n m i c r o c e l l ( a p e r t u r e v o l u m e o f 5 0 / J ) o f f u s e d s i l i c a w i n d o w s w i t h a r a n g e o f 220 to 2500 tt w a s u t i l i z e d .

A variable-beam

c o n d e n s e r s e r v e d t o attenuate the reference b e a m . A n a c t u a l w a v e l e n g t h cutoff at 230 tt w a s o b s e r v e d f o r t h e m i c r o c e l l . I n o r d e r t o l o w e r t h e Downloaded by FUDAN UNIV on January 16, 2017 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0104.ch007

cutoff p o i n t , a m a t c h e d 1-cm s t a n d a r d s i l i c a c e l l filled w i t h solvent w a s c e n t e r e d b e h i n d t h e reference b e a m attenuator.

This allowed a lower

cutoff a t 205 it. E t h a n o l , 9 5 % , w a s chosen b e c a u s e of a l o w cutoff p o i n t , 205 tt, a n d a h i g h d i s s o l u b l e c a p a c i t y f o r t h e c o m p o u n d s o f interest. T h e samples w e r e c o l l e c t e d f r o m G L C c o l u m n s b y t w o t e c h n i q u e s . A system w i t h K B r as a n absorbent was u s e d to collect G L C peaks f r o m single-column operation

with

a

flame

i o n i z a t i o n detector.

This was

adapted from a n original design for d u a l column operation ( 2 7 ) . It consisted o f c o l u m n effluent s p l i t t e r , c o l l e c t i o n t u b e , a n d f r a c t i o n collector. T h e s p l i t t i n g ratio o f t h e flow w a s 1:1.7, d e t e c t o r : o u t p u t .

T h i s system

w a s chosen f o r t h e f o l l o w i n g reasons: (a)

T h e r e is a d i r e c t G L C t o m i c r o s p e c t r o p h o t o m e t r i c

w i t h m i n i m u m intermediate handling.

operation

T h i s eliminates contamination

problems i n h a n d l i n g microgram quantities. T h e c o l l e c t i o n o f s e v e r a l peaks d u r i n g o n e c h r o m a t o g r a p h i c r u n

(b)

is possible. (c)

T h e ease o f o p e r a t i o n a n d r e p e a t a b i l i t y .

(d)

C o l l e c t e d fractions c a n b e d i s s o l v e d a n d r e c h r o m a t o g r a p h e d .

(e)

R e c o v e r i e s of 5 0 to 7 5 % h a v e b e e n o b t a i n e d .

T h e G L C peak collected o n the K B r m a y b e inserted into the m i c r o cell b y either of t w o techniques: b y p o u r i n g the p o w d e r directly into a Table I X . Common Name Atratone Simeton Prometon Simazine Propazine Trietazine Atrazine

Absorption Maxima for Some syw-Triazine Pesticides" Chemical

Name

A

2-Ethylamino-4-isopropylamino-6-methoxy-striazine 2,4-Bis(ethylamine)-6-methoxy-s-triazine 2 , 4 - B i s (isopropy l a m i n o - 6 - m e t h o x y ) - s - t r i a z i n e 2-Chloro-4,6-bis(ethylamino)-s-triazine 2-Chloro-4,6(isopropylamino)-s-triazine 2-Chloro-4-

Ultraviolet Spectrophotometry in Residue Analysis; SpectraâStructure

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